Rotary knives have various applications in continuous processes of treating or otherwise working with moving webs. For example, in an automatic transfer mechanism for a continuous winding operation, the web must be severed at a location between the new core and the full winding roll. This immediately produces a "tail" which must quickly wrap around the surface of the new core if a fold-over of the tail is to be avoided. A tail fold-over produces a bump which lasts for many wraps and which can damage the material in these wraps.
To ensure that the tail catches up with the surface of the new core so as to avoid fold-over, it is necessary that the knife fire in close proximity to the core and that the knife travel at a velocity greater than web speed. For slow web speeds, conventional cutoff knives can be fired by pneumatic or hydraulic cylinders. To achieve higher knife speeds, however, a rotary knife and one of the single revolution type is used in order to make only one cut. Rotary knives of the prior art must either be of a large diameter, which interferes with access to the core, or must employ a large drive motor and drive transmission components to achieve extremely high acceleration and deceleration rates.
A compound motion device has also been suggested wherein the knife accelerates to top speed and then swings into the cutting position. In such an arrangement the swinging device becomes a problem since a knife of reasonably small diameter makes one revolution in a very short time. The inertia of the pivoting knife about the pivot axis is very high. It also requires large components which interfere with the new core.
An alternative method for avoiding tail fold back is a die cut arrangement in which a steel rule type die is employed and in which the core is used as a platen so as to eliminate the tail in the first place. While this system is satisfactory for many installations, not all cores can be used as platens and for high web speeds it is difficult to get the knife in and out of the cutting position quickly.
In addition to the cutting situations outlined above, there exists the situation in which it is desirable to eliminate an overlapping type splice of a new roll to an expiring roll made in the course of an unwinding operation and to replace the lap splice with a butt splice. In one method of the prior art of splicing a new roll to an expiring roll in the course of an unwinding operation, the web is stopped by the use of accumulators and a zero speed butt splice is made directly on the web ends. The accumulators are very expensive and occupy a great deal of space on the line.
In a second method of butt splicing in an unwinding operation, a previously formed lap splice is cut out and the web ends are butt spliced at line speed. This method normally involves the use of a die type cutter set on a rotary armature. The cut is almost perpendicular to the direction of web travel and high stress is put on the splice as it travels through the process.